Stewart, who also snaps the ball for punts and placekicks, was signed in June and released before training camp by the Steelers. Long snapper Greg Warren has missed the brunt of the past two seasons with separate ACL surgeries. Warren has been healthy so far this summer.

Might be just giving him rest over the next couple weeks to be fresh for the season.
In games, he'll snap maybe 8-10 times on average. He may be snapping 100 times a week right now.
Obviously these aren't facts, but it could be something similar.

...or they could be worried about Warren though they likely would bring in a real replacement that has played in games if they thought Warren wasn't able to go.